1. Field of the Invention
The invention relates to a permanent magnet and a method of manufacturing the permanent magnet, and relates in particular to a permanent magnet having a metal with high coercive force diffused in the interior thereof, and to a method of manufacturing the permanent magnet.
2. Description of Related Art
Coercivity (Hc) and remanence (Br) are used as measures of the performance of permanent magnets. Coercivity is defined as the intensity of a reverse external magnetic field required to return a magnetized body to an unmagnetized state. Remanence is the magnetization that remains when the external magnetic field is zero.
When a permanent magnet is disposed on the rotor of a rotating electrical machine, it is affected by the magnetic field from the stator. That is, if the direction of the magnetic field from the stator is the reverse of the magnetization direction of the permanent magnet, the permanent magnet undergoes demagnetization in case its coercivity is small. To increase the coercivity of the surface of a permanent magnet when exposed to an external magnetic field, a metal with high coercive force is diffused from the surface towards the interior of the permanent magnet.
For example, Japanese Patent Application Publication No. 2012-39100 (JP 2012-39100 A) discloses a manufacturing method whereby the coercive force of a permanent magnet is improved. Namely, highly coercive dysprosium (Dy) or terbium (Tb) is added by grain boundary diffusion to a neodymium (Nd)-iron (Fe)-boron (B) sintered magnet, substituting Dy or Tb for Nd.
Japanese Patent Application Publication No. 2011-108776 (JP 2011-108776 A) also discloses improving coercive force by grain-boundary diffusion. The metal grains of highly coercive Dy or Tb are diffused in an Nd—Fe—B sintered magnet. In this case, it is stated that the magnetic properties of the permanent magnet are actually reduced if Dy or the like completely permeates the interior of the permanent magnet. Therefore, it is considered better if diffusive permeation of the metal grains is limited to a depth in a range of about 10 μm or more to a few mm in the surface layer.
Japanese Patent Application Publication No. 2012-43968 (JP 2012-43968 A) also discloses improving coercive force by grain-boundary diffusion. The metal grains of highly coercive Dy or Tb are diffused in an Nd—Fe—B sintered magnet. In this case, yttrium (Y), which has a smaller oxide generation energy than either Nd or Dy, is included in the magnet before diffusion. It is said that this causes deeper diffusion of Dy in the interior of the sintered body.
Japanese Patent Application Publication No. 2010-259231 (JP 2010-259231 A) discloses dividing a permanent magnet for a magnetic field pole into multiple magnet pieces, although the dividing direction of the magnet is different from that of this invention. In this case, the matrix of a permanent magnet for a magnetic field pole is made as a rectangular bar, and divided into multiple magnet pieces in the longer direction so as to control heat generation caused by eddy current in a permanent magnet for a magnetic field. The multiple magnet pieces are separated by insulating members between them, and connected so as to obtain the same shape as the original permanent magnet.
According to these documents, the surface coercivity of a permanent magnet can be increased by diffusing a highly coercive metal from the surface towards the interior of the permanent magnet. As discussed in JP 2011-108776 A, diffusion of the highly coercive metal is limited to a certain depth. Therefore, if a permanent magnet with increased surface coercivity is divided into multiple magnet parts as described in JP 2010-259231 A, part of the interior of the permanent magnet matrix, which lacks the diffused highly coercive metal, is exposed on the division surface. Demagnetization may occur when an exposed surface without increased coercivity is exposed to a strong alternating field.